Over the past years, stiffness testing of structural products has been widely used in manufacturing and building industries as an important quality concern warranting security of use of these products in the field. Stiffness characteristics of structural products are usually tested through the measurement of a parameter known as modulus of elasticity (E) also known as Young modulus, which is essentially defined as the ratio of the magnitude of a load applied to the article over the magnitude of the corresponding deformation induced to the same article as a result of the applied load. In the lumber processing industry, stiffness measurement is usually performed as part of a standard quality testing procedure known as Machine Stress Rating (MSR) in addition to the assessment of other lumber characteristics such as geometric and surface features, to comply with the requirements of specific applications such as I-beams for flooring and roof truss structures. Typically, the measured modulus of elasticity value for each piece of lumber is compared to reference threshold of increasing values associated with increasing quality of lumber, so as to assign a corresponding grade to each piece of lumber tested.
A first approach to measure the stiffness of lumber consists of using a static testing bench wherein the piece of lumber is disposed on two spaced apart support elements while a load of a predetermined magnitude is applied onto an area of the piece of lumber located between the two support elements, for measuring the corresponding deflection induced. Such basic approach is employed by the apparatus disclosed in U.S. Pat. No. 4,589,288 issued to Porter et al on May 20, 1986 which makes use of two series of parallel rolls for laterally supporting a wood panel to be tested and a loading bar capable of applying a linear load at a center area of the panel and transversally thereto, by means of a two-way cylinder for sequentially applying a first load magnitude followed by a second load of an incremented magnitude, which load magnitudes are chosen so as to involve a substantially linear portion of the deflection curve characterizing the tested panel. The applied load magnitudes are measured with a load cell and the extension or distance moved by the cylinder in applying the incremental load is either predetermined or measured in real-time. A similar testing approach is also used by the system disclosed in U.S. Pat. No. 6,053,052 issued to Starostovic on Apr. 25, 2000. Although such static approach has become considered in the wood processing industry as a standard procedure whose results are widely employed as reference values according to which MSR grades are established, in the context of on-line quality procedures, its use is limited to the testing of sampled pieces coming from the production line, and cannot be implemented as a real-time, dynamic testing procedure for all pieces being processed while they are conveyed through the production line.
A second, dynamic approach for carrying out stiffness testing consists of measuring the modulus of elasticity E of a piece of lumber while it is conveyed lengthwise, typically downstream from a lumber planer. Such dynamic stiffness testing approach is used by the apparatus disclosed in U.S. Pat. No. 3,196,672 issued to Keller on Jul. 27, 1965, which apparatus includes first and second series of rolls between which is disposed a load-measuring roll in such a manner to impart a predetermined deflection to the piece of lumber passing thereon. A third series of rolls at a location downstream from the first load-measuring roll, and a second load-measuring roll disposed between the second and third series of rolls are used to impart a second predetermined deflection onto an opposed face of the piece of lumber as compared to the face onto which the first deflection is imparted. The opposed deflection removes the effect of bow and warp naturally present in the piece of lumber. Load measurement signals are then integrated as the piece of lumber is passing through the apparatus, and a main value as an estimation of the modulus of elasticity E of the entire piece of lumber is obtained.
A similar dynamic stiffness measurement approach involving longitudinal piece conveying is also employed by the apparatus disclosed in U.S. Pat. No. 5,503,024 issued to Bechtel et al on Apr. 2, 1996, and in U.S. Pat. No. 5,564,573 issued to Palm et al on Oct. 15, 1996. While representing an improvement over the static testing approach as to the capability of these prior dynamic testing apparatus to systematically test all pieces of lumber as they are processed in the production line, the use of such apparatus is limited to industrial installations where there is sufficient available space within the production line to receive these prior art apparatus whose dimensions generally exceed the length of the longer piece of lumber to be processed.
A variant of above-mentioned dynamic stiffness testing approach is disclosed is U.S. Pat. No. 4,289,037 issued to Vinopal on Sep. 15, 1981 which describes a system making use of a conveyer for transporting wood pieces lengthwise through a first roll-based load applying device used to apply a transversal load on a central area of the wood piece located between two supporting rolls to induce a corresponding longitudinal deflection of the wood piece, means for measuring respective magnitudes of the applied load and the induced deflection, a second roll-based device for applying a load of a second magnitude on the same area of the wood piece, means for measuring respective magnitude of the second load and second corresponding deflection induced on the wood piece, and a computer for classifying the tested wood piece according to load and deflection magnitudes and to assign a grade accordingly. A similar approach for on-line stiffness testing of wood panels is disclosed in U.S. Pat. No. 5,804,738 (CA 2,220,789) issued to Bach et al on Sep. 8, 1998. The use of roll-based load applying device as taught by the above-mentioned prior patents is associated with problems related to load measurement signals stability which adversely affects consistency and reliability of stiffness estimation. The fact that a load applying roll is characterized by a loading surface that is limited to a peripheral portion of its circumference adjacent the loaded surface of the article in the conveying direction yields to such load measurement signal stability problems, especially in cases where significant vibration occurs when the article is transported on the conveyer. The ultimate effect of this limitation is to yield inconsistent stiffness estimation that may result to classification errors such as under-grading or over-grading of pieces of lumber.
An alternative approach that has been developed to comply with minimum space requirement consists of measuring stiffness characteristics while each piece of lumber is conveyed along a path in a direction parallel to the transverse dimension of the piece of lumber. Such approach is employed by the apparatus disclosed in U.S. Pat. No. 3,158,021 issued to Walters et al on Nov. 24, 1964, according to which limit bending stress of wood pieces are measured using a transverse conveyer provided on a loading station making use of two parallel lever-mounted weights disposed over the transverse conveyer so as to distribute a corresponding load onto a central area of each wood piece transversally conveyed. Such prior art apparatus carrying out a single load measurement corresponding to a single deflection measurement to obtain the desired bending stress limit measurement, the significant influence of bow and warp that are naturally present on most pieces of lumber cannot be adequately compensated according to the proposed technique.
There is still a need for testing stiffness apparatus and methods which advantageously comply with minimum space requirements imposed by industrial users of stiffness testing system, while ensuring enhanced load measurement signals stability to provide reliable and consistent stiffness estimation.